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Axel Rasmussen 59e978522d BACKPORT: FROMGIT: userfaultfd: add minor fault registration mode 
Patch series "userfaultfd: add minor fault handling", v9.

Overview
========

This series adds a new userfaultfd feature, UFFD_FEATURE_MINOR_HUGETLBFS.
When enabled (via the UFFDIO_API ioctl), this feature means that any
hugetlbfs VMAs registered with UFFDIO_REGISTER_MODE_MISSING will *also*
get events for "minor" faults.  By "minor" fault, I mean the following
situation:

Let there exist two mappings (i.e., VMAs) to the same page(s) (shared
memory).  One of the mappings is registered with userfaultfd (in minor
mode), and the other is not.  Via the non-UFFD mapping, the underlying
pages have already been allocated & filled with some contents.  The UFFD
mapping has not yet been faulted in; when it is touched for the first
time, this results in what I'm calling a "minor" fault.  As a concrete
example, when working with hugetlbfs, we have huge_pte_none(), but
find_lock_page() finds an existing page.

We also add a new ioctl to resolve such faults: UFFDIO_CONTINUE.  The idea
is, userspace resolves the fault by either a) doing nothing if the
contents are already correct, or b) updating the underlying contents using
the second, non-UFFD mapping (via memcpy/memset or similar, or something
fancier like RDMA, or etc...).  In either case, userspace issues
UFFDIO_CONTINUE to tell the kernel "I have ensured the page contents are
correct, carry on setting up the mapping".

Use Case
========

Consider the use case of VM live migration (e.g. under QEMU/KVM):

1. While a VM is still running, we copy the contents of its memory to a
   target machine. The pages are populated on the target by writing to the
   non-UFFD mapping, using the setup described above. The VM is still running
   (and therefore its memory is likely changing), so this may be repeated
   several times, until we decide the target is "up to date enough".

2. We pause the VM on the source, and start executing on the target machine.
   During this gap, the VM's user(s) will *see* a pause, so it is desirable to
   minimize this window.

3. Between the last time any page was copied from the source to the target, and
   when the VM was paused, the contents of that page may have changed - and
   therefore the copy we have on the target machine is out of date. Although we
   can keep track of which pages are out of date, for VMs with large amounts of
   memory, it is "slow" to transfer this information to the target machine. We
   want to resume execution before such a transfer would complete.

4. So, the guest begins executing on the target machine. The first time it
   touches its memory (via the UFFD-registered mapping), userspace wants to
   intercept this fault. Userspace checks whether or not the page is up to date,
   and if not, copies the updated page from the source machine, via the non-UFFD
   mapping. Finally, whether a copy was performed or not, userspace issues a
   UFFDIO_CONTINUE ioctl to tell the kernel "I have ensured the page contents
   are correct, carry on setting up the mapping".

We don't have to do all of the final updates on-demand. The userfaultfd manager
can, in the background, also copy over updated pages once it receives the map of
which pages are up-to-date or not.

Interaction with Existing APIs
==============================

Because this is a feature, a registered VMA could potentially receive both
missing and minor faults.  I spent some time thinking through how the
existing API interacts with the new feature:

UFFDIO_CONTINUE cannot be used to resolve non-minor faults, as it does not
allocate a new page.  If UFFDIO_CONTINUE is used on a non-minor fault:

- For non-shared memory or shmem, -EINVAL is returned.
- For hugetlb, -EFAULT is returned.

UFFDIO_COPY and UFFDIO_ZEROPAGE cannot be used to resolve minor faults.
Without modifications, the existing codepath assumes a new page needs to
be allocated.  This is okay, since userspace must have a second
non-UFFD-registered mapping anyway, thus there isn't much reason to want
to use these in any case (just memcpy or memset or similar).

- If UFFDIO_COPY is used on a minor fault, -EEXIST is returned.
- If UFFDIO_ZEROPAGE is used on a minor fault, -EEXIST is returned (or -EINVAL
  in the case of hugetlb, as UFFDIO_ZEROPAGE is unsupported in any case).
- UFFDIO_WRITEPROTECT simply doesn't work with shared memory, and returns
  -ENOENT in that case (regardless of the kind of fault).

Future Work
===========

This series only supports hugetlbfs.  I have a second series in flight to
support shmem as well, extending the functionality.  This series is more
mature than the shmem support at this point, and the functionality works
fully on hugetlbfs, so this series can be merged first and then shmem
support will follow.

This patch (of 6):

This feature allows userspace to intercept "minor" faults.  By "minor"
faults, I mean the following situation:

Let there exist two mappings (i.e., VMAs) to the same page(s).  One of the
mappings is registered with userfaultfd (in minor mode), and the other is
not.  Via the non-UFFD mapping, the underlying pages have already been
allocated & filled with some contents.  The UFFD mapping has not yet been
faulted in; when it is touched for the first time, this results in what
I'm calling a "minor" fault.  As a concrete example, when working with
hugetlbfs, we have huge_pte_none(), but find_lock_page() finds an existing
page.

This commit adds the new registration mode, and sets the relevant flag on
the VMAs being registered.  In the hugetlb fault path, if we find that we
have huge_pte_none(), but find_lock_page() does indeed find an existing
page, then we have a "minor" fault, and if the VMA has the userfaultfd
registration flag, we call into userfaultfd to handle it.

This is implemented as a new registration mode, instead of an API feature.
This is because the alternative implementation has significant drawbacks
[1].

However, doing it this was requires we allocate a VM_* flag for the new
registration mode.  On 32-bit systems, there are no unused bits, so this
feature is only supported on architectures with
CONFIG_ARCH_USES_HIGH_VMA_FLAGS.  When attempting to register a VMA in
MINOR mode on 32-bit architectures, we return -EINVAL.

[1] https://lore.kernel.org/patchwork/patch/1380226/

Link: https://lkml.kernel.org/r/20210301222728.176417-1-axelrasmussen@google.com
Link: https://lkml.kernel.org/r/20210301222728.176417-2-axelrasmussen@google.com
Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Chinwen Chang <chinwen.chang@mediatek.com>
Cc: Huang Ying <ying.huang@intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: Lokesh Gidra <lokeshgidra@google.com>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: "Michal Koutn" <mkoutny@suse.com>
Cc: Michel Lespinasse <walken@google.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Shaohua Li <shli@fb.com>
Cc: Shawn Anastasio <shawn@anastas.io>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Steven Price <steven.price@arm.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Adam Ruprecht <ruprecht@google.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Cannon Matthews <cannonmatthews@google.com>
Cc: "Dr . David Alan Gilbert" <dgilbert@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Mina Almasry <almasrymina@google.com>
Cc: Oliver Upton <oupton@google.com>
Cc: Kirill A. Shutemov <kirill@shutemov.name>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>

(cherry picked from commit 82a150ec394f6b944e26786b907fc0deab5b2064
https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git akpm)
Link: https://lore.kernel.org/patchwork/patch/1388132/

Conflicts: arch/x86/Kconfig
	fs/userfaultfd.c
	include/linux/userfaultfd_k.h
	include/uapi/linux/userfaultfd.h
	init/Kconfig
	mm/hugetlb.c
(Lack of userfaultfd write-protect support in 5.4 lead to all conflicts.
Resolved by carefully rebasing such that write-protect related code
doesn't get added)
Signed-off-by: Lokesh Gidra <lokeshgidra@google.com>
Bug: 160737021
Bug: 169683130
Change-Id: I43b37272d531341439ceaa03213d0e2415e04688
2025-05-18 08:08:42 +00:00
android Merge tag 'ASB-2025-03-05_11-5.4' into android13-5.4-lahaina 2025-04-12 09:31:28 +00:00
arch BACKPORT: FROMGIT: userfaultfd: add minor fault registration mode 2025-05-18 08:08:42 +00:00
block Merge tag 'ASB-2025-05-05_11-5.4' of https://android.googlesource.com/kernel/common into android13-5.4-lahaina 2025-05-18 08:00:20 +00:00
certs
crypto This is the 5.4.291 stable release 2025-03-13 14:53:52 +00:00
Documentation Merge tag 'ASB-2025-05-05_11-5.4' of https://android.googlesource.com/kernel/common into android13-5.4-lahaina 2025-05-18 08:00:20 +00:00
drivers qcedev: Fix excessive logging on DRM playback 2025-05-18 11:08:08 +03:00
fs BACKPORT: FROMGIT: userfaultfd: add minor fault registration mode 2025-05-18 08:08:42 +00:00
include BACKPORT: FROMGIT: userfaultfd: add minor fault registration mode 2025-05-18 08:08:42 +00:00
init BACKPORT: FROMGIT: userfaultfd: add minor fault registration mode 2025-05-18 08:08:42 +00:00
ipc ipc: replace costly bailout check in sysvipc_find_ipc() 2024-09-04 13:15:02 +02:00
kernel Merge tag 'ASB-2025-05-05_11-5.4' of https://android.googlesource.com/kernel/common into android13-5.4-lahaina 2025-05-18 08:00:20 +00:00
lib Merge tag 'ASB-2025-05-05_11-5.4' of https://android.googlesource.com/kernel/common into android13-5.4-lahaina 2025-05-18 08:00:20 +00:00
LICENSES
mm BACKPORT: FROMGIT: userfaultfd: add minor fault registration mode 2025-05-18 08:08:42 +00:00
net Merge tag 'ASB-2025-05-05_11-5.4' of https://android.googlesource.com/kernel/common into android13-5.4-lahaina 2025-05-18 08:00:20 +00:00
samples Merge 5.4.287 into android11-5.4-lts 2024-12-16 10:50:16 +00:00
scripts Merge tag 'ASB-2025-05-05_11-5.4' of https://android.googlesource.com/kernel/common into android13-5.4-lahaina 2025-05-18 08:00:20 +00:00
security Merge tag 'ASB-2025-05-05_11-5.4' of https://android.googlesource.com/kernel/common into android13-5.4-lahaina 2025-05-18 08:00:20 +00:00
sound Merge tag 'ASB-2025-05-05_11-5.4' of https://android.googlesource.com/kernel/common into android13-5.4-lahaina 2025-05-18 08:00:20 +00:00
techpack Merge tag 'LA.UM.9.14.r1-26000-LAHAINA.QSSI15.0' of https://git.codelinaro.org/clo/la/platform/vendor/opensource/display-drivers into android13-5.4-lahaina 2025-04-12 09:35:58 +00:00
tools UPSTREAM: selftests/vm/.gitignore: add mremap_dontunmap 2025-05-18 08:08:42 +00:00
usr
virt KVM: arm64: Ignore PMCNTENSET_EL0 while checking for overflow status 2024-12-19 18:05:04 +01:00
.clang-format
.cocciconfig
.get_maintainer.ignore
.gitattributes
.gitignore Merge tag 'ASB-2024-12-05_11-5.4' of https://android.googlesource.com/kernel/common into android13-5.4-lahaina 2024-12-17 03:24:53 +02:00
.mailmap
Android.mk
Androidbp
build.config.aarch64
build.config.allmodconfig
build.config.allmodconfig.aarch64
build.config.allmodconfig.arm
build.config.allmodconfig.x86_64
build.config.arm
build.config.common
build.config.db845c
build.config.gki
build.config.gki-debug.aarch64
build.config.gki-debug.x86_64
build.config.gki.aarch64 ANDROID: GKI: add Trimble symbol list 2025-01-16 06:41:15 -08:00
build.config.gki.x86_64
build.config.gki_kasan
build.config.gki_kasan.aarch64
build.config.gki_kasan.x86_64
build.config.gki_kprobes
build.config.gki_kprobes.aarch64
build.config.gki_kprobes.x86_64
build.config.hikey960
build.config.msm.common
build.config.msm.gki
build.config.msm.lahaina
build.config.x86_64
build.targets
COPYING
CREDITS
gen_headers_arm.bp
gen_headers_arm64.bp
gen_headers_arm64_auto.bp
gen_headers_arm_auto.bp
Kbuild
Kconfig
kernel_headers.py
LICENSE
MAINTAINERS
Makefile Merge tag 'ASB-2025-05-05_11-5.4' of https://android.googlesource.com/kernel/common into android13-5.4-lahaina 2025-05-18 08:00:20 +00:00
modules.list.msm.lahaina
OWNERS
README
README.md

README.md

How do I submit patches to Android Common Kernels

  1. BEST: Make all of your changes to upstream Linux. If appropriate, backport to the stable releases. These patches will be merged automatically in the corresponding common kernels. If the patch is already in upstream Linux, post a backport of the patch that conforms to the patch requirements below.

  2. LESS GOOD: Develop your patches out-of-tree (from an upstream Linux point-of-view). Unless these are fixing an Android-specific bug, these are very unlikely to be accepted unless they have been coordinated with kernel-team@android.com. If you want to proceed, post a patch that conforms to the patch requirements below.

Common Kernel patch requirements

  • All patches must conform to the Linux kernel coding standards and pass script/checkpatch.pl
  • Patches shall not break gki_defconfig or allmodconfig builds for arm, arm64, x86, x86_64 architectures (see https://source.android.com/setup/build/building-kernels)
  • If the patch is not merged from an upstream branch, the subject must be tagged with the type of patch: UPSTREAM:, BACKPORT:, FROMGIT:, FROMLIST:, or ANDROID:.
  • All patches must have a Change-Id: tag (see https://gerrit-review.googlesource.com/Documentation/user-changeid.html)
  • If an Android bug has been assigned, there must be a Bug: tag.
  • All patches must have a Signed-off-by: tag by the author and the submitter

Additional requirements are listed below based on patch type

Requirements for backports from mainline Linux: UPSTREAM:, BACKPORT:

  • If the patch is a cherry-pick from Linux mainline with no changes at all
    • tag the patch subject with UPSTREAM:.
    • add upstream commit information with a (cherry-picked from ...) line
    • Example:
      • if the upstream commit message is
        important patch from upstream

        This is the detailed description of the important patch

        Signed-off-by: Fred Jones <fred.jones@foo.org>

    - then Joe Smith would upload the patch for the common kernel as

        UPSTREAM: important patch from upstream

        This is the detailed description of the important patch

        Signed-off-by: Fred Jones <fred.jones@foo.org>

        Bug: 135791357
        Change-Id: I4caaaa566ea080fa148c5e768bb1a0b6f7201c01
        (cherry-picked from c31e73121f4c1ec41143423ac6ce3ce6dafdcec1)
        Signed-off-by: Joe Smith <joe.smith@foo.org>
  • If the patch requires any changes from the upstream version, tag the patch with BACKPORT: instead of UPSTREAM:.
    • use the same tags as UPSTREAM:
    • add comments about the changes under the (cherry-picked from ...) line
    • Example:
        BACKPORT: important patch from upstream

        This is the detailed description of the important patch

        Signed-off-by: Fred Jones <fred.jones@foo.org>

        Bug: 135791357
        Change-Id: I4caaaa566ea080fa148c5e768bb1a0b6f7201c01
        (cherry-picked from c31e73121f4c1ec41143423ac6ce3ce6dafdcec1)
        [ Resolved minor conflict in drivers/foo/bar.c ]
        Signed-off-by: Joe Smith <joe.smith@foo.org>

Requirements for other backports: FROMGIT:, FROMLIST:,

  • If the patch has been merged into an upstream maintainer tree, but has not yet been merged into Linux mainline
    • tag the patch subject with FROMGIT:
    • add info on where the patch came from as (cherry picked from commit <sha1> <repo> <branch>). This must be a stable maintainer branch (not rebased, so don't use linux-next for example).
    • if changes were required, use BACKPORT: FROMGIT:
    • Example:
      • if the commit message in the maintainer tree is
        important patch from upstream

        This is the detailed description of the important patch

        Signed-off-by: Fred Jones <fred.jones@foo.org>

    - then Joe Smith would upload the patch for the common kernel as

        FROMGIT: important patch from upstream

        This is the detailed description of the important patch

        Signed-off-by: Fred Jones <fred.jones@foo.org>

        Bug: 135791357
        (cherry picked from commit 878a2fd9de10b03d11d2f622250285c7e63deace
         https://git.kernel.org/pub/scm/linux/kernel/git/foo/bar.git test-branch)
        Change-Id: I4caaaa566ea080fa148c5e768bb1a0b6f7201c01
        Signed-off-by: Joe Smith <joe.smith@foo.org>
  • If the patch has been submitted to LKML, but not accepted into any maintainer tree
    • tag the patch subject with FROMLIST:
    • add a Link: tag with a link to the submittal on lore.kernel.org
    • if changes were required, use BACKPORT: FROMLIST:
    • Example:
        FROMLIST: important patch from upstream

        This is the detailed description of the important patch

        Signed-off-by: Fred Jones <fred.jones@foo.org>

        Bug: 135791357
        Link: https://lore.kernel.org/lkml/20190619171517.GA17557@someone.com/
        Change-Id: I4caaaa566ea080fa148c5e768bb1a0b6f7201c01
        Signed-off-by: Joe Smith <joe.smith@foo.org>

Requirements for Android-specific patches: ANDROID:

  • If the patch is fixing a bug to Android-specific code
    • tag the patch subject with ANDROID:
    • add a Fixes: tag that cites the patch with the bug
    • Example:
        ANDROID: fix android-specific bug in foobar.c

        This is the detailed description of the important fix

        Fixes: 1234abcd2468 ("foobar: add cool feature")
        Change-Id: I4caaaa566ea080fa148c5e768bb1a0b6f7201c01
        Signed-off-by: Joe Smith <joe.smith@foo.org>
  • If the patch is a new feature
    • tag the patch subject with ANDROID:
    • add a Bug: tag with the Android bug (required for android-specific features)

Vibrator driver for HHG device

How to merge the driver into kernel source tree

  1. Copy ${this_project}/drivers/hid/hid-aksys.c into ${your_kernel_root}/drivers/hid/

  2. Compare and merge ${this_project}/drivers/hid/hid-ids.h into ${your_kernel_root}/drivers/hid/hid-ids.h : Add the following code before the last line of this file

    	#define USB_VENDER_ID_QUALCOMM  0x0a12
	#define USB_VENDER_ID_TEMP_HHG_AKSY 0x1234
	#define USB_PRODUCT_ID_AKSYS_HHG  0x1000

  3. Merge ${this_project}/drivers/hid/Kconfig into ${your_kernel_root}/drivers/hid/Kconfig : Add the following code before the last line of this file

    config HID_AKSYS_QRD
	tristate "AKSys gamepad USB adapter support"
	depends on HID
	---help---
	Support for AKSys gamepad USB adapter

config AKSYS_QRD_FF
	bool "AKSys gamepad USB adapter force feedback support"
	depends on HID_AKSYS_QRD
	select INPUT_FF_MEMLESS
	---help---
	Say Y here if you have a AKSys gamepad USB adapter and want to
	enable force feedback support for it.

  4. Merge ${this_project}/drivers/hid/Makefile into ${your_kernel_root}/drivers/hid/Makefile : Add the following code at the end of this file

    obj-$(CONFIG_HID_AKSYS_QRD)	+= hid-aksys.o

  5. Modify your kernel's default build configuration file. Add the following two lines:

    CONFIG_HID_AKSYS_QRD=m
CONFIG_AKSYS_QRD_FF=y